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Raman amplifiers have been developed to amplify optical signals. A Raman amplifier relies upon the Raman scattering effect. The Raman scattering effect is a process in which light is frequency downshifted in a material. The frequency downshift results from a nonlinear interaction between light and the material. The difference in frequency between the input light and the frequency downshifted light is referred to as the Stokes shift which in silica fibers is of the order 13 THz.
When photons of two different wavelengths are present in an optical fiber, Raman scattering effect can be stimulated. This process is referred to as stimulated Raman scattering (SRS). In the SRS process, longer wavelength photons stimulate shorter wavelength photons to experience a Raman scattering event. The shorter wavelength photons are destroyed and longer wavelength photons, identical to the longer wavelength photons present initially, are created. The excess energy is conserved as an optical phonon (a lattice vibration). This process results in an increase in the number of longer wavelength photons and is referred to as Raman gain.
The probability that a Raman scattering event will occur is dependent on the intensity of the light as well as the wavelength separation between the two photons. The interaction between two optical waves due to SRS is governed by the following set of coupled equations:                     ⅆ                  I          S                            ⅆ        z              =                            g          R                ⁢                  I          S                ⁢                  I          P                    -                        α          S                ⁢                  I          S                                        ⅆ                  I          P                            ⅆ        z              =                            -                                    λ              S                                      λ              P                                      ⁢                  g          R                ⁢                  I          S                ⁢                  I          P                    -                        α          P                ⁢                  I          P                    
where Is is the intensity of the signal light (longer wavelength), Ip is the intensity of the pump light (shorter wavelength), gR is the Raman gain coefficient, xcexs is the signal wavelength, xcexp is the pump wavelength, and xcex1s and xcex1p are the fiber attenuation coefficients at the signal and pump wavelengths respectively. The Raman gain coefficient, gR, is dependent on the wavelength difference (xcexs-xcexp) as is well known in the art.
As is well understood in the art, SRS is useful for generating optical gain. Optical amplifiers based on Raman gain are viewed as promising technology for amplification of WDM and DWDM telecommunication signals transmitted on optical fibers. Until recently, Raman amplifiers have not attracted much commercial interest because significant optical gain requires approximately one watt of optical pump power. Lasers capable of producing these powers at the wavelengths appropriate for Raman amplifiers have only come into existence over the past few years. These advances have renewed interest in Raman amplifiers.
FIG. 4 depicts a prior art arrangement of optical system 40 which includes a Raman amplifier. Optical system 40 includes optical signal source which generates an optical signal to be detected by detector 44. For example, telecommunication providers utilize wavelengths within the C Band (1530 to 1565 nm) and L Band (1570 to 1610 nm) to provide channels to carry information optically. Additionally, it is anticipated telecommunication providers may also begin to utilize wavelengths in the S Band (1430 to 1530 nm) and the XL Band (1615 to 1660 nm). Accordingly, the optical signal may comprise one or more wavelengths within these bands. Detector 44 is disposed at some appreciable distance from optical signal source 42. Raman source 41 provides a Raman pump. Raman source 41 provides the Raman pump to multiplexer 43. Multiplexer 43 causes the Raman pump to enter optical fiber 45 which also carries the optical signal generated by optical signal source 42. Due to SRS, the optical signal experiences Raman gain at the desired wavelength(s) in fiber 45.
External cavity diode lasers (ECDL""s) are most typically used to narrow the linewidth of the laser. In this context, linewidth refers to or measures the width of the spectral content of the output of a laser diode. By utilizing an external cavity, the linewidth of a laser can be reduced by many orders of magnitude. An example of an external cavity laser is provided in U.S. Pat. No. 5,319,668.
The reduction in linewidth of an ECDL can result in an accompanying process that is referred to as Brillouin scattering. Brillouin scattering is analogous to Raman scattering. The primary differences are that, in lieu of an optical phonon, an acoustic phonon is generated, the Stokes shift in silica fibers is 10 GHz instead of 13 THz, and the Brillouin gain coefficient is about 2 orders of magnitude larger. It will be appreciated that if a typical ECDL is used as a Raman pump source, Brillouin scattering will backscatter the pump light in competition with stimulated Raman scattering. Specifically, this backscattering prevents the pump light from propagating down the length of the fiber to stimulate the Raman process. Accordingly, typical ECDL""s are not suitable for Raman amplifier pump source applications.
The present invention is directed to a system and method for operating an external cavity laser to obtain a linewidth controlled output. In some embodiments, the system and method modify an incoherently beam combined (IBC) laser to achieve the desired linewidth control. In some embodiments, laser diodes of an IBC laser are modified to cause the laser diodes to operate in a coherence collapse regime (a non-linear region of laser operation defined by feedback effects) by selecting etalon surface reflectivity of the diodes relative to feedback received from the external cavity. By operating the diodes in the coherence collapse regime, the laser diodes are caused to have significantly broadened linewidth due to non-linear effects. In other embodiments, the linewidth of the laser diodes is broadened by utilizing a phase modulator. The relatively broad linewidth of embodiments of the present invention, in turn, can be used to adapt an external cavity device such as an IBC laser for use as a Raman pump.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention.